The present invention relates to the art of tube bending apparatus and, more particularly, to improvements in connection with bending a tube to a desired bend angle.
While the present invention finds particular utility in connection with portable, motor driven tube benders and, accordingly, will be described in detail hereinafter in conjunction therewith, it will be appreciated that the invention is applicable to bending other workpieces such as rods for example and is likewise applicable to bending apparatus which is either manually operated or motor driven and not necessarily portable in character.
Tube bending apparatus of the character to which the present invention is directed is well known and, basically, is comprised of mandrel and shoe or forming member components supported for relative angular displacement about a bending axis. The mandrel has a bending groove extending about the bending axis, and a tube positioned between the mandrel and forming member is adapted to be bent to a desired bend angle during relative angular displacement of the mandrel and forming member about the bending axis. Relative angular displacement between the mandrel and forming member can be achieved in a number of different ways. In a hand held bender, for example, the mandrel is mounted on one handle and the forming member is pivotally supported relative to the mandrel and associated with a second handle by which the forming member is displaced relative to the mandrel. In other hand operated and in some motor driven benders, the mandrel is mounted on a support and the forming member is mounted on the support for angular displacement relative to the mandrel and is so displaced by a manually actuated handle or through a motorized drive arrangement. In yet other motor operated benders, the forming member is supported against displacement and the mandrel is angularly displaced relative thereto such as by a drive motor.
Tube benders of the foregoing character are generally adapted to be operable to selectively bend a tube to a bend angle of up to 180.degree.. Most frequently, the bend angle is 45.degree., 90.degree. or 180.degree.. In any event, it is extremely difficult to obtain accuracy with respect to bending a tube to a given bend angle. In this respect, tube benders heretofore available most often rely on indicia provided on the apparatus to visually indicate the angular degree of a bend to the operator. Regardless of whether or not the apparatus is manually operated or motor driven, considerable care must be taken to control the relative displacement between the mandrel and forming member in order to obtain an accurate bend as well as accuracy with respect to successive bends to the same or different bend angles. Further adding to the difficulty in connection with achieving accurate bends is the fact that the inherent resilience of the tube material can provide a spring-back effect, whereby overbending relative to a desired bend angle is necessary in connection with obtaining the desired bend angle. The degree of spring-back varies with the diameter of the tube, the wall thickness of the tube and/or the tube material. Therefore, even if a degree of overbending is taken into account in connection with providing the bend angle indicia on the apparatus, inaccuracy with respect to a given bend angle is likely to result from the foregoing variables, whereby more time and effort can be required on the part of the operator in an effort to obtain an accurate bend angle.
Manually adjustable stop arrangements have also been provided in connection with tube bending apparatus in an effort to obtain accurate bends. Such stop arrangements are shown, for example, in U.S. Pat. Nos. 3,236,082 to Beck et al and 3,417,590 to Ensley. However, such adjustable stops rely in part on the accuracy of positioning of one of the stop components by the operator and, while positively terminating a bending operation after a predetermined and fixed angular displacement between the mandrel and forming member, do not assure accuracy with respect to obtaining a given bend angle when the adjustable stop member is moved from one setting to another and then back to the one setting. Moreover, such adjustable stops do not compensate for the different degrees of overbending required as a result of the variances in tube diameter, wall thickness and tube material referred to above. Accordingly, the operator must either guess at the amount of overbend required and position the adjustable stop member accordingly, or attempt a number of overbends by trial and error in order to ultimately obtain the desired bend angle. In either event, an undesirable amount of time and effort on the part of the operator is required and, moreover, there is no assurance of being able to obtain the necessary degree of overbend from one bending operation to another if the required degree of overbend changes as a result of the foregoing variable factors.